1. Field of the Invention
The present invention relates to a display device and a manufacturing method thereof.
2. Description of the Related Art
In a display device such as a liquid crystal display device, the development of a display device which uses bottom-gate-type thin film transistors formed by using poly-crystalline silicon has been in progress. In case of the bottom-gate-type thin film transistor, a gate electrode is formed on an insulation substrate, and a first insulation film, a semiconductor film and a second insulation film are formed on the gate electrode in that order. The semiconductor film has a channel region, and the channel region overlaps with the gate electrode as viewed in a plan view. An impurity region doped with impurities which impart an n-type, for example, to the region is formed at a portion of the semiconductor film around the channel region. The channel region is doped with impurities which impart a type (p type, for example) different from the type imparted to the impurity region for controlling characteristics of the thin film transistor such as a threshold voltage. Doping of impurities into the channel region of the semiconductor film is performed using an ion implantation method or the like, for example. In this case, the channel region is doped with impurities such that the maximum level of concentration of the impurities in the film thickness direction of the semiconductor film is arranged at or below a center portion of the semiconductor film. To study the distribution of impurity concentration in the channel region, the impurity concentration of a portion of the channel region in the vicinity of a surface thereof which is in contact with the second insulation film does not exceed impurity concentration of a portion of the channel region in the vicinity of the surface thereof which is in contact with the first insulation film.